The 39-year-old actor portrays William "Billy" Bulger - the law-abiding brother of Boston mobster and FBI informant James "Whitey" Bulger - in Black Mass, which premiered at the Odeon Leicester Square as part of the BFI London Film Festival.

He took the night off from playing Hamlet on stage for the gala screening and was joined by his wife Sophie Hunter, as well as Hollywood star Johnny Depp, Depp's wife Amber Heard as well as the film's director Scott Cooper and producer John Lesher.

Cumberbatch said on the red carpet: "This role didn't give me a taste of politics.

"American politics is a whole different thing. I had to do quite a bit of learning what a state senate and a president of a state senate actually does and what it means."

He added: "It (the film) gave me a taste for Boston though. It's an amazing place. I had a great time doing this and prepping for it."

Cumberbatch, best known for his portrayal of sleuth Sherlock Holmes in the BBC drama, admitted there is an "added pressure" in playing real-life characters, saying: "I did feel pressure especially those who are still alive, still in the vicinity and who everyone knew I was playing. It is a bit of a pressure but I think it's one of those challenges I enjoy. The accent was the thing I was most nervous about."

He explained: "That was the whole point - to lose myself for me and the audience. Hopefully within the first five minutes they forget about the baggage of the other characters I've played."

The crime drama, directed by Crazy Heart's Scott Cooper, is based on the book Black Mass: The True Story Of An Unholy Alliance Between The FBI And The Irish Mob by two former Boston Globe reporters Dick Lehr and Gerard O'Neill.

Depp also attempted to contact Bulger, now 84 and is serving two life sentences for racketeering and the murder of 11 people, who declined to meet the actor and give his blessing to the film.

The gangland boss was finally captured in 2011 after evading the authorities for 16 years.

The Pirates Of The Caribbean star said he was fine with the snub: "I understood. I understood very well. It was a long shot because the idea of some guy playing him in a film must be very weird.

"But the response I got was very gentlemanly. He said: 'I respectfully decline only based on the fact that I am not particularly a big fan of the book.' Which is understandable. How could he be?"

The film's director said there was no doubt in his mind that Depp was perfect for the role of Whitey.

Critics have hailed his performance and there is speculation he could receive another Oscar nomination.

Cooper said: "He's an actor who is daring and bold and he has played real-life characters in the past. He isn't afraid to completely shed his personality to transform into the man you see on screen - cold, calculating and violent."

Black Mass, which also stars Joel Edgerton and Dakota Johnson, opens in UK cinemas on November 27.